The present disclosure is directed to non-invasive quantitative measurements of blood flow parameters and, in particular, the disclosure relates to a system and method for determining quantitative measurements of blood flow parameters in vascular networks using medical imaging data, such as time-resolved, three-dimensional (3D) angiographic images, referred to as four-dimensional (4D) angiographic x-ray data.
Since the introduction of angiography beginning with the direct carotid artery punctures of Moniz in 1927, there have been ongoing attempts to develop angiographic techniques that provide diagnostic images of the vasculature, while simultaneously reducing the invasiveness associated with the procedure. In the late 1970's, a technique known as digital subtraction angiography (DSA) was developed based on real-time digital processing equipment. Due to steady advancements in both hardware and software, DSA can now provide depictions of the vasculature in both 2D and volumetric 3D formats. Three-dimensional digital subtraction angiography (3D-DSA) has become an important component in the diagnosis and management of people with a large variety of central nervous system vascular diseases as well as other vascular diseases throughout the body.
In recent years competition for traditional DSA has emerged in the form of computed tomography angiography (CTA) and magnetic resonance angiography (MRA). CTA is a less invasive technique but has lower spatial resolution. It is not time-resolved unless the imaging volume is severely limited. The images are not isotropic and secondary reconstruction yields degraded spatial resolution. CTA is also somewhat limited as a standalone diagnostic modality by artifacts caused by bone at the skull base and as well as the contamination of arterial images with opacified venous structures. Further, CTA provides no functionality for guiding or monitoring minimally-invasive endovascular interventions.
Significant advances have been made in both the spatial and the temporal resolution qualities of MRA. Currently, gadolinium-enhanced time-resolved MRA (TRICKS) is widely viewed as a dominant clinical standard for time-resolved MRA. TRICKS enables voxel sizes of about 10 mm3 and a temporal resolution of approximately 10 seconds. Advancements such as HYBRID highly constrained projection reconstruction (HYPR) MRA techniques, which violate the Nyquist theorem by factors approaching 1000, can provide images with sub-millimeter isotropic resolution at frame times just under 1 second. Nonetheless, the spatial and temporal resolution of MRA are not adequate for all imaging situations and its costs are considerable. Furthermore, the spatial and temporal resolution is substantially below other methods, such as DSA.
The recently-introduced, four-dimensional (4D) DSA techniques can use rotational DSA C-arm imaging systems controlled with respect to a particular injection timing so that there is time dependence in the acquired reconstructed 4D volumes. As described in U.S. Pat. No. 8,643,642, which is incorporated herein by reference, a 3D DSA volume can be used as a constraining volume to generate a new 3D volume that contains the temporal information of each projection. As in 3D DSA, a mask rotation without contrast is followed by a second rotation during which contrast is injected. The process creates a series of time resolved 3D angiographic volumes that can be updated, for example, every 1/30 of a second.
Thus, the above-described systems and methods have improved over time and, thereby, provided clinicians with an improving ability to visualize the anatomy of the vessels being studied. Of course, blood flow through vessels is dynamic and ideally both the dynamics of blood flow and the structural features of the vessels could be used by a clinician to deduce whether or not there was an abnormality. Currently, with ever increasing spatial and temporal resolution, the clinician has been provided with clearer and more accurate information about the geometry (i.e., anatomy) of the vessels. Unfortunately, assessment of the equally important dynamics of blood flow through the vasculature still depends upon a qualitative assessment gained from visualization of a contrast bolus as it passes through the vessels. As such, while the deductions made by the clinician about the structural dynamics and function of the vessel (i.e. anatomy) have correspondingly improved, even the best deductions about the circulatory dynamics (e.g. blood flow and velocity) are still qualitative and thus inherently limited.
As such, many have worked to create systems and methods to derive quantitative measures of complex, in vivo hemodynamics. Some systems rely on interventions to acquire direct quantitative measurements, such as using catheters or the like. Of course, interventional systems and methods are less desirable than non-interventional systems and methods, if the same quality and quantity of information is available.
As such, some have developed systems and methods that use imaging data to derive blood flow measurements. The imaging modalities include ultrasound, MR, 2D DSA, and 4D-DSA. With the advancement of hardware and software in these imaging modalities, some quantitative in vivo blood flow measurements have been made over large 3D volumes in a timely manner. These rich data sets can provide functional information related to the anatomical and hemodynamic properties of complex vessel networks. However, due to the complexity of in vivo blood fluid dynamics, the accurate quantitation of flow parameters is a non-trivial task and has not approached that of interventional systems and methods.
Therefore, it would be desirable to provide systems and methods that simplify the quantitation of hemodynamic parameters, such as flow, area, mean velocity, and the like, in a way that is consistent, repeatable, and understandable to the clinician.